drm/radeon/kms: Fix apparent typo in legacy CRTC memory bandwidth calculation.

While investigating the cause of CRTC FIFO underruns, I noticed that when
converting the memory bandwidth calculation from the userspace X driver code,
an instance of '8.0' was apparently accidentally converted to '80'.

Signed-off-by: Michel Dänzer <daenzer@vmware.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>